With the rapid advancement of automotive industry, there has been an rapid progress of powertrain in modern vehicles. With the problems of engine emission and fuel supply, Hybrid Electric Vehicles (HEV) are widely used.
An HEV is a car including two or more power sources, thus there may be one or more wheel drive system. The common power sources comprises fuel, battery, fuel cell, solar cell and compressed gas, and the common drive system comprises combustion engines, motors and turbo engines, etc. A majority of HEVs use both combustion engine system and electric motor system. Most of such HEVs perform better than normal combustion engine vehicles on fuel efficiency and acceleration characteristics.
There exists an HEV choosing the main electric motor or the combustion engine to output power to gearbox, then the gearbox transfers the power to wheels. The main electric motor is supplied with power by a high-voltage battery pack. A gearbox need constant lubricating oil from an oil pump so as to perform the function of clutch, lubrication and minor cooling. Generally speaking, a main oil pump and an auxiliary oil pump are disposed between the oil tank and the gearbox. In case of a low environment temperature, the high-voltage battery pack and main electric motor can not work normally, then the combustion engine starts to work and the auxiliary oil pump starts to pump oil into gearbox, instead of the main oil pump, thus the vehicle is able to start to run at an environment of low temperature.
However, in the environment of low temperature, the viscosity of oil increases rapidly, which leads to a hard start of the auxiliary oil pump. Engineers sometime use high-power pump to solve such problem, but the size of the oil pump has to be big, which embarrasses the vehicle design and consume precious electrical power in low temperature.